Abstract
The geochemical characterization of Novokrivoyrog metavolcanics (2.2 Ga) and Krivoy Rog iron ores (1.8 Ga) in Ukraine represent an important tool for the understanding of their genesis and tectono-magmatic evolution. The petrological classification of the metavolcanics on SiO2/(Zr-TiO2) and (Zr-TiO2)/(Nb/Y) Harker-type diagrams shows similarities to subalkaline andesitic basalts. An additional classification of the basalts on TAS (Na2O + K2O/SiO2) and AFM (FeO-MgO-Na2O + K2O) diagrams exhibits a variable magmatic character from calc-alkaline to tholeiitic. The distribution of High Field Strength Elements, (HSFE: Ti, Zr, Y, Hf, Nb), V, Cr, and Rare Earth Elements (REE) in most of the rocks is close to calc-alkaline basalts (CAB) and can be compared to Precambrian mid-ocean ridge basalts (MORB) where high thermal (>250°C) basaltic alteration is intensive under pH conditions between 2 and 4. These contributed to the deposition of the Krivoy Rog BIFs. Indeed REE distribution patterns of the BIFs suggest that they can be subdivided into shales and shaly BIFs (rich in LREEs since their detrital and clastic inputs are much higher) with (La/Yb)N > 1 as indication of clastic inputs; chert and cherty BIFs showing positive Eu anomaly with (La/Yb)N 1 emphasizes post-depositional effects related to the enrichment of light REEs over heavy REEs with a positive Eu anomaly. The distribution of REE patterns of Krivoy Rog BIFs can finally be compared to Precambrian iron formations of mixed submarine hydrothermal fluids and seawater origin which correspond to the MORB signature of the Novokrivoyrog metavolcanics.
Highlights
Sedimentary iron ores are found in Precambrian geological sequences as simple iron formations (IFs) or banded iron formations (BIFs) associated with green stones or metavolcanic rocks
The geochemical characterization of metavolcanic rocks of Novokrivoyrog may answer the questions whether they resulted from rifting processes or they originated from a subduction related volcanic arc or what could be their contribution to the deposition of Krivoy Rog banded iron formations (BIFs)
The profile of Krivoy Rog BIF exhibits three different iron ore units (Figure 2): a lower iron ore unit which is composed of two quartzite and two schist horizons associated with carbonate rocks; a medium unit which presents two schist horizons of green schist metamorphic grade separated by a considerable quartzite layer, and; an upper iron ore unit which is mostly of greenschist metamorphic grade indications for higher metamorphic facies are available through rare occurrences of quartz-biotite-epidote and quartz-biotite-cummingtonite-garnet associations
Summary
Sedimentary iron ores are found in Precambrian geological sequences as simple iron formations (IFs) or banded iron formations (BIFs) associated with green stones or metavolcanic rocks. The different fractionation properties of intermediate (MREEs), light (LREES), heavy (HREEs) Rare Earth Elements combined with redox sensitive elements such as Ce and Eu when showing positive or negative anomalies may give an indication to the physicochemical environment of REEs partitioning, the sources, the mobilization and precipitation of iron. These aspects are considered in this article with regards to the Krivoy Rog iron formations in Ukraine.
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